Belt feeders

Process industries frequently need to weigh and control the flow rate of bulk material for optimum performance of such devices as grinders or pulverizers, or for controlling additives, eg, to water suppHes. A scale can be installed in a belt conveyor, or a short belt feeder can be mounted on a platform scale. Either can be equipped with controls to maintain the feed rate within limits by controlling the operation of the device feeding the material to the conveyor. Direct mass measurement with a nuclear scale can also be used to measure and control such a continuous stream of material. 

Fig. 9. Preferential flow channel caused by (a) a constant pitch screw feeder and (b) a belt feeder.

Belt Feeders. Belts ate used to feed overlong slotted openings. Typically, belt feeders are used to handle friable, coarse, fibrous, elastic, sticky, or very cohesive soHds. Because belts are available in widths up to 2.74 m and unrestricted lengths, such feeders can be designed for very large outlets. 

Fig. 11. Schematics of belt feeder interfaces to provide increasing capacity, where (a) shows side elevation and (--) is the side view of the belt feeder on...

Fig. 12. Optimum belt feeder interface (a) elevation views, where (----------) is the belt feeder on its centerline, and (b) plan view.

Some drawbacks to the weigh belt are that it is a zero-reference device and thus needs frequent caHbration (re-zeroing). Buildup on the belt and roUers affects accuracy and operation, as does belt tension and dusty or floodable materials. Flexible connections are required to isolate the feeder from upstream and downstream equipment, unless the belt feeder/weigh idler concept is used. 

Measuring process parameters on full-scale plants is notoriously difficult, but is needea for control. Usually few of the important variables are accessible to measurement. Recycle of material makes it difficult to isolate the effects of changes to individual process units in the circuit. Newer plants have more instrumentation, including on-line viscosimeters [Kawatra and Eisele, International ]. Mineral Processing, 22, 251-259 (1988)], mineral composition by on-line X-ray fluorescence, belt feeder weighers, etc., but the information is always incomplete. Therefore it is helpful to have models to predict quantities that cannot be measured while measuring those that can. 

Belts have been used successfully under slot openings as long as 30 m (100 ft), with a constant slot width of 205 mm (8 in). Provisions should be made for field adjustment of the space between the sldrt and the belt to provide uniform flow along the entire length. Since the minimum distance between the sldrt and the belt should allow the largest particle to pass under, veiy long belt feeders are hmited to the finer sohds. 

When solution must be pumped, consideration should be given to use of holding tanks between the dry feed system and feed pumps, and the solution water supply should be controlled to prevent excessive dilution. The dry feeders may be started and stopped by tank level probes. Variable-control metering pumps can then transfer the alum stock solution to the point of application without further dilution. Means should be provided for calibration of the chemical feeders. Volumetric feeders may be mounted on platform scales. Belt feeders should include a sample chute and box to catch samples for checking actual delivery with set delivery. Gravimetric feeders are usually furnished with totalizers only. Remote instrumentation is frequently used with gravimetric equipment, but seldom used with volumetric equipment. 

The plug flow contacting of phases may be accomplished in many ways by countercurrent flow as in blast furnaces and cement kilns [Fig. 26. ( )], by cross-flow as in moving belt feeders for furnaces [Fig. 26.1(6)], or by cocurrent flow as in polymer driers [Fig. 26.1(c)]. 

Figure 3.7. Solids feeders with variable speed drives, (a) Rotary vane (star) feeder with variable speed drive, (b) Horizontal screw feeder, (c) Belt feeder taking material from a bin with an adjustable underflow weir, (d) Rotary plate feeder Rate of discharge is controlled by the rotation speed, height of the collar, and the position of the plow, (e) Continuously weighing feeder with variable speed belt conveyor.

Belt feeder (feeder breaker) crawler-mounted surge bin often equipped with a crusher or breaker and used in room-and-pillar sections positioned at the end of the section conveyor belt. It allows quick discharge of a shuttle car. It sizes the coal, and a built-in conveyor feeds it at an appropriate rate onto the conveyor belt. 

Balls are charged uniformly over the cross section at the top of a shaft furnace by means of a shuttle belt feeder while the stock level is kept constant by continuously removing hardened pellets from the bottom of the shaft. 

Figure 8 shows a commercial fat apphcator. The pellets enter through a hopper, are metered by a loss-in-weight belt feeder to cascade through a spray box, and then are mixed while being conveyed by mixing screw. A less expensive fat... 

Fuel was loaded onto a flat belt feeder metering onto a high speed auger injecting into the lower bed. Fuel feed rate was controlled by varying the feeder belt speed and the height of the fuel bed on the belt. Average fuel feed rate was determined from the total fuel consumption and duration of each test. Instantaneous feed rate was estimated from the belt speed. 

---